1. Field of the Invention
The present invention relates to techniques for color space conversion in which each color component of a color in three-dimensional color space is interpolated only in a single dimension.
2. Description of the Related Art
Color space conversion, that is, conversion of color coordinate values in a first three-dimensional color space to corresponding color coordinate values in a second three-dimensional color space, is a necessary part of today""s computerized color image processing. Perhaps the most common example of color space conversion is a conversion from RGB color space to CMY color space so that an image which is displayed on a computer monitor (which requires color values for each of red, green and blue intensities) can be printed on a color printer (which requires values for each of cyan, magenta and yellow ink intensities). Other examples of color space conversion include conversions from CIELAB color space to RGB or CMY color space, and vice-versa.
One popular technique for color space conversion involves a look-up table combined with interpolation for color values not identically found in the look-up table. More specifically, to populate a look-up table, input color space is divided into cubes which may be the same size as illustrated in FIG. 1 or different sizes as illustrated in FIG. 2. The look-up table is populated with color coordinate values by measurement and/or calculation for each grid point (i.e., the corner of each cube). The output of any input color is found by interpolating among the eight grid points which form the corners of a cube containing the input color. Tri-linear interpolation and tetrahedral interpolation, as described in U.S. Pat. No. 4,477,833 (Clark, et al.) and U.S. Pat. No. 4,275,413 (Sakamoto, et al.), are widely-used interpolation techniques.
Although interpolation techniques currently used for color space conversion provide highly accurate results, even when the look-up table is only populated sparsely, the calculations needed to perform such interpolations are complex and time-consuming. Accordingly, a need exists for color conversion techniques which utilize simplified interpolation so as to decrease the time required for each color conversion.
It is therefore an object of the present invention to address the foregoing need by providing a color space conversion technique with simplified interpolation in which each color component of a color in three-dimensional color space is interpolated only in a single dimension. Particularly with a color look-up table which is generously populated, such as with 16 or 33 gradations on each color coordinate axis, the inventor herein has discovered that interpolation in only a single dimension yields much the same visual results as more complex tri-linear or tetrahedral interpolation, provided that the proper grid points are selected for single dimension interpolation.
Thus, in one aspect of the invention, for a target color whose color coordinate values are given in a first three-dimensional color space, corresponding color coordinate values in a second three-dimensional color space are obtained by reference to a look-up table by which color coordinate values in the first three-dimensional color space may be converted to color coordinate values in the second three-dimensional color space. The look-up table includes plural grid-arranged points corresponding to respectively different colors in the first three-dimensional color space, and provides color coordinate values for the colors in the second three-dimensional color space. One common grid point is selected from the look-up table, the common grid point being selected based on proximity to the target color such that the common grid point is one of the eight grid points that define corners of a cube which encloses the target color. Preferably, the common grid point always lies at the corner of the enclosing cube that has the same relative orientation to the target color, no matter where in the look-up table the target color and the enclosing cube are positioned, and the common grid point also preferably lies on the neutral gray axis when the enclosing cube includes the neutral gray axis of the look-up table. A first grid point adjacent to the common grid point is selected in a direction related to the first color coordinate, and one-dimensional interpolation is performed between the common grid point and the first grid point based on the input color coordinate value so as to obtain a first color coordinate value for the target color of the second three-dimensional color space. For example, in one representative embodiment, if conversion is desired from RGB color space to CMY color space, to obtain a cyan coordinate value of the target color, the first grid point is selected in a direction that varies in red only, since changes in red value are most related to changes in cyan value. The same procedure is applied for selecting second and third grid points adjacent to the common grid point in directions related to the second and third color coordinates, respectively, whereafter interpolation in one dimension is performed between the common grid point and the first and second grid points based on the inputted color coordinate values so as to obtain second and third color coordinate values for the target color in the second three-dimensional color space.
By virtue of the foregoing arrangement, because each color component in three-dimensional color space is interpolated only in a single dimension, rapid interpolation can be performed yielding converted color coordinate values more quickly than in conventional techniques. Moreover, since the direction of one-dimension interpolation is selected differently for each color component value and in relation to the color coordinate itself, the difference between colors converted according to the present invention and those converted according to more accurate conventional interpolation techniques is minimal, and nearly imperceptible for generously-populated color look-up table such as those with 16 or 33 grid intervals.
This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiment thereof in connection with the attached drawings.